1. Field of the Invention
The present invention is related to a method of forming a CMOS transistor.
2. Description of the Prior Art
The industrial circles are used to reducing device dimensions to improve the performance of metal-oxide semiconductor (MOS) transistors. However, this method encounters difficulties with high-expenses and technical bottlenecks in recent years. For these reasons, the industrial circles seek other methods to improve MOS transistor performance. And accordingly, a highly noticed method is to utilize the material characteristics to cause strain effect on MOS transistors.
In order to increase the driving current of a complementary metal-oxide semiconductor (CMOS) transistor including a p-type MOS (PMOS) transistor and an n-type MOS (NMOS) transistor, the industrial circles develop a strained-silicon technique, which uses unique processes or lattice constant discrepancy to increase driving current. The strained-silicon technique substantially includes a substrate-strained based method and a process-induced strain based method. The substrate-strained based system is performed with a strained-silicon substrate or a selective epitaxial growth process that results in lattice constant discrepancy. The process-induced strain based method is performed with several unique processes to form a strained thin film upon a surface of the MOS transistor that exert tensile stress or compressive stress upon the MOS transistor. Both of the strained-silicon techniques introduce strain into the channel region and reduce carrier mobile resistance thereby improving carrier mobility and MOS transistor performance.
During the fabrication processes for forming a CMOS transistor, a hard mask, such as a silicon nitride layer, is formed covering on the NMOS transistor or the PMOS transistor, and so that the exposed transistor is treated in the following process. Moreover, a phosphoric acid treatment is performed to remove the silicon nitride layer. However, a substrate loss occurs during the phosphoric treatment, and particularly to the substrate having high a concentration of dopants therein. Therefore, it is considerable to improve the conventional processes of the strained-silicon technique and to maintain the performance of the devices formed by the strained-silicon technique.
For the reasons above, the industry circles try to develop a method of strained-silicon technique to fabricate CMOS transistors and improve CMOS transistor reliability.